As expected, co-sputtering zirconia proved to be an efficient way to frustrate crystallization in tantala thin films, allowing for a substantial increase of the maximum annealing temperature \(T_a\) , hence for a decrease of coating mechanical loss. The lowest average loss, \(\overline{\varphi _c} = 1.8 \times 10^{-4}\) , was measured on IBS sample MLD2018 with cation ratio Zr/(Ta+Zr) = 48.5 \(\pm \)  0.4 % annealed at 800 \(^\circ \) C. This value has to be compared to \(\varphi _c = (2.3-3.4) \times 10^{-4}\)  of tantala-titania layers in current GW detectors [1]}, [2]}.
